FkpA is a periplasmic protein of E. coli exhibiting both molecular chaperone and cis-trans peptidyl-prolyl isomerase activities. Native FkpA is localized to the periplasmic space as a 245 amino acid homodimer in E. coli by virtue of a signal sequence directing secretion to the periplasm. The chaperone activity of FkpA resides in the first 120 amino acids of the mature protein and the peptidyl-prolyl isomerase activity resides in amino acids 121 to 245. The effect of FkpA overexpression in the periplasm on periplasmic yields of antibody single chain variable fragments (scFvs) has been studied. See Ramm and Pluckthun, J Biol Chem, 275:22, 17106-17113 (2000).
Skp is a 17-kDa periplasmic chaperone for outer membrane protein assembly in E. coli that facilitates proper folding of outer membrane protein intermediates and helps to maintain their solubility. Skp functions as a homotrimer and belongs to a family of cavity-containing chaperones that bind their substrates in the cavity, thereby providing a suitable environment for proper folding and protecting the substrate from aggregation. The effect of co-expression of Skp in the cytoplasm and a Fab antibody fragment in the bacterial cytoplasm has been studied. See Levy et al., Protein Expr Purif. 23(2):338-47 (2001).
A common problem with expressing heterologous proteins, such as antibodies, in prokaryotic systems is aggregation of the heterologous protein to such an extent that a significant portion of the protein is insoluble or nonfunctional. The methods and materials of the invention provide a solution to this problem.